10 Reaons to exclude viruses from the tree of life...

A Review of: Moreira and Lopez-Garcia. Ten Reasons to exclude viruses from the tree of life. Nature. Vol 7. 2009.

This is an opinion article in which the authors argue quite passionately that to include viruses in the tree of life would mean that it would become a ‘tree of genes from multiple origins’ instead as, without cells, viruses are nothing more than ‘inanimate complex organic matter’.

The first point they argue is that viruses are not alive. Many scientists, including Aristotle, have proposed definitions of life but as of yet none of them can include viruses. The first definition postulates that an organism is alive if it can ‘self-replicate and self-maintain’. As viruses lack any kind of carbon metabolism, they are excluded from this definition. The second involves ‘self-replication and evolution’ of the organism. Again, viruses cannot be included as they require a cell to carry out these tasks. Under this second definition it could be argued that computer viruses are alive as they can be designed to produce copies of themselves with slight changes in their code which, by extension can be seen as mutation and therefore evolution.

The fact that viruses are polyphyletic and have no ancestral viral lineages is also argued as there is no single gene shared by all viruses or viral lineages and therefore viruses have various evolutionary origins. Common protein motifs in distinct viral lineages have been found leading to the theory of a common ancient origin that predates the last common ancestor of cellular organisms: the Cenancestor. The authors argue here that horizontal gene transfer and convergence are more likely explanations.

The existence of a genetic membrane provides strong evidence that all modern cells are derived from a single common ancestor as it can only be formed by splitting pre-existing membranes. Its absence in viruses is additional evidence for their polyphyletic origin because viral constituents are synthesized de novo at each viral infection cycle and shows that viral lineages lack structural continuity.

Some viruses have been found to contain metabolic and translation genes. The authors argue that these are the result of horizontal gene transfer from a cellular origin. They describe viruses as a reservoir of cellular genes that can be transferred between different hosts and could therefore play a part in cellular adaptation and evolution but in no way help in the modification of pre-existing genes or the creation of completely new genes, hypothesized by some scientists.

Many people believe that because viruses are so small they must be very old. Moreira and Lopez-Garcia argue that structural simplicity neither implies antiquity or primitiveness but is a consequence of parasitism. Viruses are subject to strong selective pressures to keep a minimal genome size in order to have faster reproduction rates, which is a major force preventing complexification.

They conclude that viruses are not alive and should not be included in the tree of life but that they had and continue to have a significant role in the evolution of life on earth. Being abundant and comprising a major selective pressure that exerts strong control on the populations of many cellular organisms’ means that they are an important source and means of maintaining biodiversity.

The passion and opinions of the authors comes across very clearly in this article. However they do appear to be a tad closed minded about certain topics at times, dismissing reasonable hypothesis and explanations rather rashly. This article does appear to have caused somewhat of a stir in the scientific world which at times is great in provoking people to get passionate about their ideas and research- (There have been a couple of reply articles). However, I wonder if Moreira and Lopez-Garcia would stand by their arguments in this article after doing a little reading about all the new discoveries of Megaviruses and the information surrounding them…?